U. S. Pat. No. 4,950,915, issued Aug. 21, 1990, corresponding to German Patent Publication (DE) 3,706,765 A1, (Spies et al.), published on Sep. 15, 1988, describes an impact sensor for a motor vehicle. The known impact sensor is arranged in a sensor housing and activates a passive safety system in response to an impact of the motor vehicle against an obstacle, whereby, for example, an airbag is inflated or a safety belt is pulled tight, etc. The known impact sensor comprises a piezosensitive acceleration detector in a sensor housing, an evaluating circuit to provide an output signal, a trigger circuit for activating the safety system in response to the output signal, and a testing circuit for testing its performance efficacy. During the testing operation, the trigger circuit is separated from the safety system. As a part of the testing circuit, an electro-acoustic or electro-mechanical converter is arranged in the sensor housing and is acoustically or mechanically coupled with the piezoelectric acceleration detector or sensor.
German Patent Publication (DE) 2,207,831 A1 (Brede et al.), published Aug. 23, 1973, discloses an impact sensor which is provided with a testing circuit for testing the performance efficacy of the impact sensor, whereby impulse amplitudes from the sensor have such a magnitude that given threshold values can be tested.
Another conventional safety system is described in an article entitled "Airbag and Belt Pretensioner Provide Increased Safety For Driver and Front Passenger", by Brambilla, published in "Automobiltechnische Zeitschrift", ATZ Nr. 84 (1982) 2, pages 77 to 83. FIG. 10 of this reference discloses a block circuit diagram with additional components for the processing of the signal produced by a single acceleration sensor. Since there is only one sensor there can be no mutual testing between at least two sensors in the above mentioned conventional safety system.